1. Field of the Invention
The present invention relates to an optical pickup apparatus and a method for adjusting its optical axis. More particularly, the present application relates to an optical pickup apparatus having a solid-state laser having a resonator as a light beam source, and a method for adjusting its optical axis.
2. Background of the Invention
An output light beam emanated from a light beam source, such as a solid-state laser, a gas laser etc. having a resonator, has an extremely narrow angle of dispersion of e.g. several milliradians, in distinction from a light beam emitted from a semiconductor laser element, and is a light beam approximate to a collimated beam having a small beam diameter. For irradiating e.g. an optical disc, with a light beam converged by an objective lens, a light beam incident on the objective lens is converged more satisfactorily if the incident light beam has a certain width. Therefore, it is necessary to enlarge the diameter of the narrow light beam. Consequently, with an optical pickup apparatus employing a solid-state laser or a gas laser, an optical device, known as a beam expander, consisting of a combination of plural lenses, is employed for creating a light beam of a larger beam diameter.
FIG. 1 schematically shows an arrangement of an optical pickup apparatus which employs the beam expander and which constitutes the background art for the present invention.
In FIG. 1, a reference numeral 101 is a laser light radiating device, more specifically, a solid-state laser device employing a laser medium such as KTP(KTiPO.sub.4). A mirror 102 deflects the laser beam radiated from the laser light radiating device 101 by 90.degree.. A beam expander 103 is made up of a combination of two concave lenses. A reference numeral 104 is a diffraction grating for separating the light radiating device into at least three light beams. A polarization beam splitter 105 separates the light beam emanated from the laser light radiating device 101 from a light beam reflected by an optical disc 109. A collimator lens 106 collimates the light beam enlarged in diameter by the beam expander 103. A reference numeral 107 is a quarter wave plate for converting the light beam generated by the laser light beam radiating device 101 into a linear polarized light beam and converting the return beam from the optical disc 109 into a circular polarized light beam. An objective lens 108 converges the light beam collimated by the collimator lens 106 on the optical disc 109. The optical disc 109 carries information signals recorded in the optically readable form, that is, as embossed pits or changes in reflectivity.
A reference numeral 110 is a multiple lens for converging a light beam separated by the polarization beam splitter 105 on a photodetector 111. The multiple lens 110 is made up of a single lens or a single optical element exhibiting a light converging operation and an optical element for detecting focusing errors.
The light beam radiated from the laser light beam radiating device 101 is bent by 90.degree. by the mirror 102 to fall on the beam expander 103. The light beam incident on the beam expander 103 is enlarged in diameter to proceed to the collimator lens 106 through the grating 104 and the polarization beam splitter 105. The collimator lens 106 collimates the light beam enlarged in diameter. The collimated light beam proceeds to the objective lens 108 through the quarter wave plate 107. The objective lens 108 radiates the incident collimated light on the optical disc 109 so as to be converged thereat. The light beam reflected by the optical disc 109 is re-incident on the polarization beam splitter 105 through the objective lens 108, the quarter wave plate 107 and the collimator lens 106. The reflected light beam is reflected substantially totally by the reflecting surface of the polarization beam splitter 105 so as to be radiated on the photodetector 111 through the multiple lens 110.
Meanwhile, the optical axis of the beam expander 103 is fixed against inadvertent movement once it is adjusted into alignment with the optical axis of the collimated light beam from the collimator lens 106. As a result, for aligning the optical axis of the beam expander 103 with that of the laser light beam radiating device 101, it is necessary to adjust the vertical deflection of the light beam from the mirror 102 towards the beam expander 103 as well as the position of the laser light beam radiating device 101. Specifically, two degrees of freedom of rotation about x and z axes and one degree of freedom of translation along x axis need to be accorded to the mirror 102, while one degree of freedom along z axis needs to be accorded to the laser light beam radiating device 101, as indicated by arrows in FIG. 1. These two degrees of freedom of rotation and one degree of freedom of translation are adjusted for aligning the optical axis of the beam expander 103 with that of the laser light beam radiating device 101. However, such adjustment, above all, the adjustment of the rotation, is difficult to perform, so that the optical pickup apparatus itself becomes bulky in size to hinder the reduction in size of the device.